1. Field of the Invention
This invention relates to highly durable pneumatic tires used under heavy loads and more particularly to a highly durable pneumatic tire comprising a carcass body formed by a number of laminated bias laid plies each including rubberized organic textile cords.
2. Description of the Prior Art
Such kind of tires are mainly used for a truck, trailer, construction vehicle, agricultural vehicle, industrial vehicle, air plane or the like.
Such kind of tire, that is, a tire comprising a carcass body formed by a number of laminated bias laid plies each including rubberized organic textile cords, has a number of advantages, but has a few disadvantages. Among these disadvantages, the important disadvantage consists in that the number of plies of the carcass must considerably be increased in order to give the tire a strength which is required for counterbalancing the conditions subjected to the tire when it is used. This point will now be described in a more concrete manner.
In a nylon cord of 1,260 denier/2 strands (1,260 d/2), which has most popularly been used at present, for example, the number of plies of the carcass usually mounts up to 10, 20 and exceeds 40 for a tire having a large size. It is clear that the use of such large number of laminated carcass plies provides the disadvantage that production efficiency becomes bad and the manufacturing cost is increased if compared with a steel radial tire which makes use of only 1 ply for a considerably large size tire.
In order to eliminate such drawback, an easy method of decreasing the number of plies while maintaining the strength necessary for the tire has been proposed. In such method, a cord diameter is made large so as to increase the number of filaments correspondingly and hence increase the strength per one cord. For example, if use is made of a cord of 1,890 d/2 instead of the cord of 1,260 d/2, the strength of the cord is increased up to 3/2 times larger and hence the number of plies is descreased to about .mu. times smaller. In order to maintain a necessary space between adjacent cords, the number of cords per unit length must be decreased so that the number of plies does not become 2/3 times smaller as is expected by calculation. As a result, it is possible to reduce the number of necessary plies from 30 to the order of 22.
However, it has been found out that if the number of carcass plies is decreased according to the above mentioned method, a vital failure, that is, a cord breaking up failure is liable to be induced in the tire.
It is a matter of course that the cord breaking up failure means a tire failure in which the ply cords become cut owing to certain reasons. As a result, the cord breaking up failure seldom occurs in the tire when it is used under proper condition, but sometimes occurs when the internal pressure in the tire becomes extremely low or when the tire is subjected to overload or when the tire is subjected to sudden shocks. In addition, the cord breaking up failure is induced when the strength of the carcass of the tire becomes insufficiently small owing, for example, to the erroneous selection of the tire size to be used.
Experimental tests have shown the result that, if the carcass is the same in strength, the use of a cord having a large diameter ensures a frequent occurrence of the cord breaking up failure if compared with the use of a cord having a small diameter.
Experimental tests and investigations on the above mentioned drawback which has been encountered with the prior art techniques, that is, on a contradictory fact that the use of a cord having a large diameter for the purpose of decreasing the number of carcass plies and hence improving the productivity of the tire results in an inevitable occurrence of the cord breaking up failure, have demonstrated the result that the above mentioned drawback can effectively be eliminated by making large the number of twists of the carcass ply cords which are located at portions where the cord breaking up failure is easily induced. The invention is based on such recognition.